MUSEUM OF COMPARA.TIVK ZOOLOGY. 71 



portion of their constituent parts as in their size and degree of diller- 

 entiation in different individuals. 



It is quite probable that some of the differences in thickness between 

 the retinas of different individuals is due to the fact that measurements 

 have been made on sections not quite meridional in direction. Care has 

 been taken, however, in each instance to avoid this source of error. liut 

 in the case of the one giving a thickness 0.145 mm. greater than that of 

 the normal retina in Clevelandia, the sections are so cut that none are 

 quite meridional. As may be gathered from Figure 13, this eye is so 

 flattened in its axial direction that the retina is almost disk-shaped ; and 

 from this fact it was possible so to cut the retina that none of the sections 

 would be entirely perpendicular to its surface. The sections are so cut 

 obliquely, though the deviation from the perpendicular is certainly not 

 sufficient to account for the great difference in thickness that is shown. 

 But the difference in proportion between corresponding layers in different 

 retinas cannot be explained, even in part, as due to artificial causes. In 

 one of the specimens 19 mm. long, the ratio between the inner reticular 

 layer and the whole retina is I : 7, while in the other it is 1 : 13. 



From the measurements here given alone, it might be concluded that 

 there is a gradual increase in thickness, and a constantly advancing dif- 

 ferentiation in the retina, with increase in the size and age of the animals. 

 Such a conclusion is not warranted, however, when the whole number of 

 specimens and stages that have been studied by me is considered, though 

 it must be admitted that, in view of the obviously wide range of in- 

 dividual variation, the number of specimens examined has not been suf- 

 ficiently large to justify an unqualified denial that such is the case. All 

 that can be said with positiveness is, that, notwithstanding the fact that 

 the thickest and most fully differentiated retina has been found in a 

 specimen much larger — and therefore presumably older — than the' 

 smallest studied, yet several still larger individuals have shown retinas 

 thinner and less differentiated than those of the smallest individuals ; 

 and, further, that in one instance at least one of the smallest individuals 

 shows in the distinctness of the outer reticular layer as great a degree of 

 differentiation as any retina examined. It would be a very interesting 

 and significant thing, if, owing to a retardation in development, differen- 

 tiation of the retina should continue thi-oughout the entire life of these 

 fishes ; since we know quite well that normally the fish eye becomes func- 

 tional and differentiated at an early period in development (Balfour and 

 Parker, '82, pp. 371 and 384 ; Ryder, '84, p. 500 ; Hoffmann, '83). 



It is exceedingly desirable to ascertain what law, if any, controls the 



